FIG. 7 illustrates a conventional metal mold 51 for compression-forming a preform.
The metal mold 51 for compression forming includes a cavity metal mold 52 which is a female mold, a core metal mold 53 which is a male mold and a slide insert metal mold 54 which is a split mold.
The cavity metal mold 52 has a cavity 56 formed therein and an upwardly facing small protuberance 57 formed surrounding the upper portion of the cavity 56. Further, a ring-like guide ring 58 is provided surrounding the cavity metal mold 52 in concentric with the cavity metal mold so as to slide. An inner circular truncated conical surface 59 having a diameter increasing upward is formed in the upper inner circumferential surface of the guide ring 58.
On the other hand, the core metal mold 53 has the core metal mold 53 that is fixed to a support member 61 that can be moved up and down. The core metal mold 53 is surrounded by the slide insert metal mold 54 which can be opened and closed in the right-and-left direction. The slide insert metal mold 54 is mounted on the support member 61 so as to be opened and closed in the right-and-left direction. A fitting portion (pressure-maintaining portion) 62 is formed in the lower inner circumferential surface of the slide insert metal mold 54 to thereby form an outer circumferential surface of nozzle portion of the preform while an outer circular truncated conical surface 60 having a diameter decreasing downward is formed in the lower outer circumferential surface thereof.
Described below is the forming operation by using the above metal mold 51 for compression-forming the preform.
At the initial position of the metal mold 51 for compression forming shown in FIG. 7A, the cavity metal mold 52 and the core metal mold 53 are arranged being spaced apart in the up-and-down direction, and a molten resin 64 is contained in the cavity 56.
In the mold-clamping step shown in FIG. 7B, the core metal mold 53 moves down into the cavity 56, and the molten resin 64 nearly fills a space defined by the cavity metal mold 52 and the core metal mold 53. At the same time, the slide insert metal mold 54, too, moves down to come in contact with the guide ring 58. Accordingly, the molten resin 64 in the cavity flows into the space defined by the core metal mold 53 and the slide insert metal mold 54. At this moment, the fitting portion 62 of the slide insert metal mold 54 starts sliding relative to the small protuberance 57 of the cavity metal mold 52 as indicated by an arrow X.
At the end of the mold clamping shown in FIG. 7C, the core metal mold 53 moves down to the lowermost end position. The space defined by the cavity metal mold 52, core metal mold 53 and slide insert metal mold 54 assumes the same shape as the preform that is to be formed and is filled with the molten resin.
At this moment as indicated by an arrow Y, sliding of the fitting portion 62 of the slide insert metal mold 54 relative to the small protuberance 57 of the cavity metal mold 52 ends. As described above, the fitting portion 62 slides against the small protuberance 57 so as to be in mesh absorbing the contraction of volume and making it possible to obtain a preform for blow forming without strain.
[Patent document 1] JP-A-2003-159739